1. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly to a high electron mobility transistor (which will be hereinafter abbreviated to HEMT).
2. Description of the Prior Art
A HEMT, a kind of FET, employing a n-AlGaAs/GaAs selectively doped hetero junction has now been developed to realize a very high speed computer and a high speed signal processing system. In such a HEMT, a crystal region (e.g., undoped GaAs) for allowing travel of electrons and a crystal region (e.g., Si doped n-AlGaAs) for supplying electrons are separated from each other by a hetero junction, and scattering of electrons by donor impurities is reduced to thereby increase an electron mobility and improve a high-speed performance.
FIG. 1 schematically shows a device structure of a AlGaAs/GaAs HEMT in the prior art. An undoped GaAs layer 11 as an active layer is formed on a semi-insulating GaAs substrate 10. An i-AlGaAs layer 12 is formed on the undoped GaAs layer 11. An Si doped n-AlGaAs layer 13 as an electron supplying layer is formed on the i-AlGaAs layer 12. An Si doped n-GaAs layer 14 is formed on the n-AlGaAs layer 13. Further, a gate electrode 15 ,a source electrode 16 and a drain electrode 17 are formed on the n-GaAs layer 14. In FIG. 1, a hatched portion under the source electrode 16 and the drain electrode 17 is an ohmic region.
The formation of a current channel (shown by a broken line in FIG. 1) for realizing an FET operation is based on the phenomenon that the electrons supplied from the donor impurities added in the n-AlGaAs layer 13 as the electron supplying layer are moved to the undoped GaAs layer 11 as the active layer and are accumulated in a portion of the undoped GaAs layer 11 in the vicinity of a junction interface between the n-AlGaAs layer 13 and the undoped GaAs layer 11. This current channel is a so-called two-dimensional electron channel.
There is a tendency that a gate length L.sub.g has increasingly been shortened, so as to realize a high performance of the HEMT. However, the reduction in the gate length L.sub.g causes a short channel effect such that the two-dimensional electrons exude toward the substrate to reduce a mutual conductance g.sub.m and accordingly deteriorate the characteristics of the HEMT. To prevent the short channel effect, the following three techniques have conventionally been proposed;
(A) To substitute the undoped GaAs layer as the active layer by a p.sup.- -GaAs layer. PA1 (B) To make the active layer into an InGaAs strain layer channel PA1 (C) To provide an i-AlGaAs layer in the undoped GaAs layer as the active layer and form a hetero junction.
In technique (A), the electrons in the active layer are scattered by the impurities existing in the active layer to reduce the electron mobility. Further, the number of two-dimensional electron carriers is also reduced.
In technique (B), a lattice constant of the InGaAs layer is different from that of the n-GaAs layer, and a strain of the InGaAs layer increases with an increase in thickness of the InGaAs layer. As a result, lattice defects in the InGaAs layer increase to reduce the electron mobility. Accordingly, the thickness of the InGaAs layer is limited to a critical thickness of 15 to 20 nm, and the InGaAs layer cannot be thickened. However, in the InGaAs layer having such a critical thickness, the number of two-dimensional electron carriers is reduced.
In technique (C), the two-dimensional electron gas is scattered in the vicinity of the hetero junction interface between the undoped GaAs layer and the i-AlGaAs layer to reduce the electron mobility, causing the generation of noise.